I was usually forced to use the larger images provided and then edit them from there. It really works out quite well as when you shrink down the larger ones some of the detail is lost, so having a super high res image at start is not always needed
Best bet for some of the more common ships is just to do an image search and find one that is suitable and edit that. Also means that your image is even more different than the stock ones. Of course for some of the ships the only images are black and white cartoon images or SWC unique ones.
For any image the art team/users modeled there is a possibility that they would render up a larger version for you, but its not done very often and most of us charge for the service. Anything not done by us has to be found elsewhere on the internet.